In recent years, attention has been focused on photoelectric converters that utilize the photoelectric effect in semiconductor to convert light into electric power, and they have been under intensive development. Among the photoelectric converters, a silicon-based thin-film photoelectric converter can be formed on a large-area substrate of glass or stainless steel at a relatively lower temperature, and hence cost reduction thereof can be expected.
Such a silicon-based thin-film photoelectric converter generally includes a transparent electrode layer, at least one photoelectric conversion unit, and a back electrode layer that are stacked successively on a transparent insulating substrate. Here, the photoelectric conversion unit generally includes a p-type layer, an i-type layer, and an n-type layer stacked in this order or reverse order, where the i-type layer for photoelectric conversion occupies a main part of the unit. Thus, a unit including an amorphous i-type photoelectric conversion layer is referred to as an amorphous photoelectric conversion unit, while a unit including a crystalline i-type layer is referred to as a crystalline photoelectric conversion unit.
The photoelectric conversion layer serves as a layer that absorbs light and generates electron-hole pairs. Generally, in a silicon-based thin-film photoelectric converter, an i-type layer in a pin junction serves as the photoelectric conversion layer. An i-type layer, which is the photoelectric conversion layer, occupies a substantial film thickness of a photoelectric conversion unit.
Ideally, the i-type layer is an intrinsic semiconductor layer that contains no conductivity-type determining impurities. However, a layer containing a slight amount of impurities is referred to as a substantially i-type layer as long as it has a Fermi level approximately at the middle of its forbidden band, because such a layer can function as an i-type layer in a pin junction. The substantially i-type layer is generally deposited without adding conductivity-type determining impurities to a source gas. In this case, the substantially i-type layer may contain conductivity-type determining impurities in an allowable concentration range that does not affect the function of the i-type layer. Alternatively, the substantially i-type layer may be deposited with intentional addition of a slight amount of conductivity-type determining impurities in order to cancel influence of impurities originating from an atmosphere or an underlayer on the Fermi level.
When amorphous silicon or an amorphous silicon alloy is used for the photoelectric conversion layer, it is usual to deposit a layer of hydrogenated amorphous silicon or a hydrogenated amorphous silicon alloy that contains 5-20 atomic % hydrogen. As is well known, since amorphous silicon containing no hydrogen includes defects caused by unpaired bonds (dangling bonds) at a density as high as 1019-1020 cm−3, such amorphous silicon cannot be used for a semiconductor device such as a photoelectric converter. In contrast, since hydrogenated amorphous silicon in which dangling bonds are terminated by hydrogen atoms has a defect density lowered to 1015-1017 cm−3, it can be used for a semiconductor device such as a photoelectric converter. A hydrogenated amorphous silicon alloy containing an element such as carbon, oxygen, nitrogen, or germanium in addition to silicon can also has a defect density lowed by hydrogen atoms and thus it can also be used for a semiconductor device such as a photoelectric converter.
As a method of improving conversion efficiency of the photoelectric converter, there is known a photoelectric converter that adopts a stacked-type structure in which at least two photoelectric conversion units are stacked. In this method, a front side unit that includes a photoelectric conversion layer having a larger optical forbidden bandwidth is disposed on a light incident side of the photoelectric converter, and to rearward thereof, rear side units including their respective photoelectric conversion layers having smaller forbidden bandwidths are successively disposed in decreasing order of the bandwidth, so that incident light in a wider wavelength range can be converted photoelectrically. Accordingly, incident light can efficiently be used to improve conversion efficiency of the entire device. (In the present application, a photoelectric conversion unit disposed relatively nearer to a light incident side is referred to as a front side unit, and another unit disposed farther from the light incident side and adjacent to the front side unit is referred to as a rear side unit.)
Among the stacked-type thin-film photoelectric converters, the one including an amorphous photoelectric conversion unit and a crystalline photoelectric conversion unit stacked therein is referred to as a hybrid-type photoelectric converter. In the hybrid-type photoelectric converter, amorphous silicon can photoelectrically convert light in a wavelength range approximately up to 800 nm on a longer-wavelength side, while crystalline silicon can photoelectrically convert light in a wider wavelength range approximately up to 1100 nm, so that it become possible to effectively achieve photoelectric conversion of incident light in a wider wavelength range.
In either of the amorphous silicon single-unit photoelectric converter and the above-described hybrid-type photoelectric converter, it is preferred to make the photoelectric conversion layer as thin as possible from a viewpoint of improvement in productivity, i.e., cost reduction. Accordingly, there is generally used a structure that utilizes a so-called optical confinement effect, in which a layer having a refractive index lower than that of the photoelectric conversion layer is disposed to rearward of the conversion layer to effectively reflect light having a specific wavelength. Disposing the layer to rearward of the photoelectric conversion layer means that the layer may be placed in contact with the rear side of the photoelectric conversion layer, or may be placed on the rear side of another layer disposed on the rear side of the photoelectric conversion layer.
As a method of more effectively utilizing the optical confinement caused by the low refractive index layer described above, there is a method of forming an intermediate transmissible-reflective layer between the thin-film photoelectric conversion units in the stacked-type photoelectric converter, wherein the intermediate transmissible-reflective layer is made of a conductive material having a lower refractive index as compared to materials of the conversion units. The intermediate transmissible-reflective layer can be designed to reflect light in a shorter wavelength range and transmit light in a longer wavelength range, so that light can more effectively be converted photoelectrically in each of the thin-film photoelectric conversion units. For example, in the case that the intermediate transmissible-reflective layer is inserted between the front side amorphous silicon photoelectric conversion unit and the rear side crystalline silicon photoelectric conversion unit in the hybrid-type photoelectric converter, electric current generated by the front side unit can be increased without increase in thickness of the amorphous silicon photoelectric conversion layer in the front side unit. Furthermore, in the case of including the intermediate transmissible-reflective layer, the thickness of the amorphous silicon photoelectric conversion layer can be made smaller, as compared to the case of not including the intermediate transmissible-reflective layer, to obtain the same current value. Therefore, it becomes possible to avoid property deterioration of the amorphous silicon photoelectric conversion unit, where the deterioration is caused by optical degradation (Sraebler-Wronsky effect) that is more significant with increase in thickness of the amorphous silicon layer.
Patent Document 1 describes a stacked-type photoelectric converter in which an n-type Si1-xOx layer is used as a low refractive index layer. The n-type Si1-xOx layer is characterized in that it has an oxygen concentration of at least 25 atomic % and at most 60 atomic % and includes silicon-rich phase parts in an amorphous silicon-oxygen alloy phase, so as to achieve both of electric conductivity and a low refractive index. Note that the term “silicon-rich” literally means a high silicon concentration. Accordingly, inclusion of silicon-rich phase parts means a state including partial phase parts of higher silicon concentration. The n-type Si1-xOx layer has a refractive index of 1.7-2.5 and a dark conductivity of 10−8-10−1 S/cm. Furthermore, it is described that the silicon-rich phase parts preferably include silicon crystal phase parts because it is considered that the silicon crystal phase parts provide current paths therethrough in the thickness direction of the n-type Si1-xOx layer and contribute to formation of favorable ohmic contact. Alternatively, it is also preferable that the silicon-rich phase parts include doped amorphous silicon parts, because either of n-type amorphous silicon and p-type amorphous silicon sufficiently doped with impurities makes it possible to obtain a film having a resistivity low enough to form ohmic contact, as is well known.
Furthermore, Patent Document 1 describes a structure in which a p-type amorphous silicon carbide layer/an i-type amorphous silicon photoelectric conversion layer/an n-type microcrystalline silicon layer/an n-type Si1-xOx layer/a p-type microcrystalline silicon layer/an i-type crystalline silicon photoelectric conversion layer/an n-type microcrystalline silicon layer are successively stacked, and the n-type Si1-xOx layer is used as an intermediate transmissible-reflective layer in the stacked-type photoelectric converter. In other words, Patent Document 1 describes that insertion of the n-type microcrystalline silicon layer between the i-type amorphous silicon photoelectric conversion layer and the n-type Si1-xOx layer is effective for reducing contact resistance at the interface and improving the fill factor (FF) of the photoelectric converter.    Patent Document 1: Japanese Patent Laying-Open No. 2005-045129